A tomographic method (layer image recording method) is generally used to record in particular three-dimensional medical image data using an imaging system. The imaging system is for example a computed tomograph (CT), an x-ray C-arm, in particular an angiograph or a Single Photon Emission Computed Tomograph (SPECT). The medical image data is obtained by measuring projection images of a body region of a patient in temporal succession one after another from different viewing angles. To measure these projection images, the medical imaging system has a measuring arrangement. In the case of a computed tomograph for example this measuring arrangement comprises an x-ray source and an x-ray detector, which are supported in a movable manner in a gantry.
The actual medical image data is calculated from the projection images during the course of a so-called back-projection. Since the projection images are captured over a specific time period, motion of the examined body region is necessarily registered as well. While voluntary patient motion can be largely avoided by securing the patient or by administering an anesthetic, this is not possible for motion stimulated by the vegetative nervous system. This relates for example to motion of the measured body region, associated with breathing, the heartbeat or peristaltic motion of an intestine.
When calculating the image data from the projection images, so-called motion artifacts result, which to some degree significantly impair the clinical benefit of the calculated data. If for example a stent is to be inserted into a coronary vessel of the heart of a patient, when the heart is measured using CT, motion causes blurring of the imaged coronary vessel, which makes it difficult to select an appropriate stent for the intervention.
To improve the quality of the image data several methods have been developed, of which an overview can be found in U.S. Pat. No. 6,708,052 B1. It is thus possible to capture the projection data in very rapid succession. Reducing the so-called acquisition time when measuring the projection data also reduces the occurrence of motion artifacts. However the shortening of the acquisition time is subject to technical limits, in particular if larger body regions are to be measured at once. Also the simultaneous use of a number of imaging systems of the same type, each capturing a part of the body region to be examined, can only be of limited assistance here.
So-called ECG gating provides a different route, measuring the ECG of the heart while the projection images are being captured. Then only the projection images corresponding to a specific heart phase are used to calculate the image information. A further approach is to implement so-called ECG triggering, with which projection images are only measured when the heart is in a specific heart phase. ECG gating and ECG triggering have the disadvantage that they only correct heart motion. Gating also takes into account a patient's breathing.
The specialist article by M. Prümmer et al, “Cardiac C-arm CT: Efficient motion correction for 4D-FBP, 2006 IEEE Nuclear Science Symposium Conference Record”, p. 2620 ff. describes a computational method for motion correction. Projection images of a body region to be examined, determined as a function of time using CT, are captured first. A sequence of medical image data is then calculated from the projection images. The so-called FDK method is used in combination with ECG gating here. The time-based medical image data thus determined, also referred to as 4D data in the specialist literature, contains information about the time-based deformation of the measured body region. The approach adopted by M. Prümmer et al in their work is that of calculating a displacement vector field from the time-based image data, said displacement vector field containing the change in all volume elements or voxels of the captured body region at any time. This displacement vector field is used to correct the originally measured projection images in respect of their location information. This corrected projection data then undergoes a further back-projection. The result is three-dimensional medical image data, in which the motion of the body region in question is corrected. This allows motion artifacts to be largely avoided in the three-dimensional medical image data. However this method is computationally very complex, requiring a high level of computation power.
U.S. Pat. No. 5,287,276 A discloses a motion correction method for computed tomography image data, in which, while the projection images are being recorded, ultrasound is used to detect the chest motion of the patient due to breathing and the detected motion is taken into account in the back-projection of the projection images.